1. Field of the Invention
The present invention relates to an electronic control device for temperature control and more particularly to an electric control device for multiple stage heating and/or cooling.
2. Description of the Prior Art
In cooling or heating large areas, a heating or cooling system i.e., temperature control system, must necessarily have a capacity sufficiently large to accomplish the desired temperature change. However, as the temperature approaches the predetermined desired temperature, the large system capacity may be unable to cease its heating or cooling effect in sufficient time to prevent the temperature of the area from overshooting the desired temperature. Thus, it is desirable to reduce capacity as the temperature approaches the desired temperature.
One method of reducing the capacity of a control system is to literally turn off components thereof. For example, a large cooling unit may have two compressors, and by turning one compressor off, the capacity is reduced. Thus, a temperature control system might have a first stage which turns on and off a first compressor, and a second stage which turns on and off a second compressor. Alternatively, a system might have two separate cooling devices which are turned on and off by the two stages.
Some applications of temperature control systems require that if the temperature of the controlled area is above a desired temperature, a cooling unit of the system is turned on, and that if the temperature is below a desired temperature, a heating unit of the system is turned on. For systems having large capacities, multistage heating and cooling units are required.
Typical temperature control systems generally comprise one or two stages, with the staging sequence controlled by complex circuitry. For example, when both stages of a two stage system are selected for heating, or are selected for cooling, stage two deactivation precedes stage one deactivation. When one stage is selected for heating and the other for cooling, one stage is disabled when the other is on. Due to the complex circuitry used to implement the staging operation and sequence of multistage systems, it becomes increasingly difficult to add additional stages and necessitates selling the device as a unit. Furthermore, prior art devices do not have the capability of starting with a single stage and adding a second stage if the need later arises.
In many applications for temperature control systems it is desirable to deactivate the subsequent (secondary) stages at temperatures offset from the temperature selected in the first (primary) stage. For example, in a control system having four stages, in which stages 1 and 2 heat, and stages 3 and 4 cool, the desired temperature is selected in stage 1. Stage 2, the second heating stage, might be selected to turn off at a number of degrees before stage 1 to prevent overshoot. Similarly, the cooling stages would also be turned off at temperatures relative to stage 1. By using offset temperatures, if the desired temperature, defined by the primary stage deactivation temperature, is changed, the temperatures at which the secondary stages deactivate are also changed, thereby insuring that overshoot will not occur.
Prior art devices typically provide each stage with a desired temperature range (hysteresis) defined by start and stop temperatures. The temperature range helps insure that the heating or cooling device does not cycle rapidly on and off about the specific level of the desired temperature.
A desirable electronic temperature control device would have a primary set point as the start or stop temperature, and be able to provide that temperature to secondary stages. The secondary stages would combine the primary set point with a secondary offset to produce a secondary set point (stop temperature) and the staging sequence should be dependent only on the selected set point temperatures. Each primary stage would be capable of driving more than one secondary stage. Furthermore, each stage should be disposed within an individual module, to facilitate the addition of stages.